A day in Tunnel town is 8 hours long, so you will see the day/night transition after 8 hours of gameplay time.  The day/night cycle pauses when you’re not playing, even though crops keep growing and your other burrow activities continue.  We designed the game this way so that players would get to experience both daytime and nighttime in their burrows, even though they might only play the game during the day.  So if you recently played Tunnel Town and it just switched to night, it will remain nighttime for your next 8 hours of playtime.